This application relates generally to radio based position location systems and in particular to a generalized method which is applicable regardless of the number of base station antennas, and which compensates for unknown delays, phase shifts, and frequency shifts in received signals.
It is known that widely deployed radio communications network equipment such as cellular and paging system base stations can be used to provide position information for remote subscriber units. Such a position location system includes multiple base stations arranged in a predetermined pattern over a large region and spaced apart at relatively large distances, typically on the order of a few miles. To determine the location of an object within the region among the base stations, plural base stations receive a signal, such as a cellular phone signal, from the object. By analyzing the phase of the signals received, the location of the object is estimated.
Typical systems employ one of two common position location approaches. One of the approaches is called direction finding (DF) or angle of arrival and the other is known as pseudo ranging (PR) or time difference of arrival (TDOA). The direction finding method uses an antenna array such as a phased array at each base station to receive the signal from the object and determine its angle of arrival at the base station. By analyzing the difference in phase of the signal received at each antenna in the array, each of the plural base stations generates an estimate of the direction to the object. The object's location is estimated to be at the point of intersection of directional lines projected from each of the plural base stations at the computed angular directions.
In pseudo ranging systems, for each pair of base stations, the difference in time of arrival of the signal from the object at each base station is computed from the phase of the received signals. This time difference defines a hyperbola for each independent pair of base stations. The point at which the hyperbolas intersect provides an estimate of the location of the object.
Both of these common approaches to position location suffer from inherent inaccuracies. Since the phased arrays of antennas cannot precisely determine the angle of arrival of the signal, the direction finding approach actually does not result in several lines intersecting at one common point. Rather, the lines intersect at several points forming a region within which the object should be located. This region can be quite large depending upon certain variables such as elevation, signal strength, etc. The pseudo ranging approach is also inherently inaccurate since multiple hyperbolas do not intersect at the same point. This and other inaccuracies also result in determining a region in which the object may be located, rather than a precise position location. See, for example, Joseph P. Kennedy, et al., Passive High Accuracy Geolocation System and Method, U.S. Pat. No. 5,317,323, issued May 31, 1994.
At least one position location system has applied both direction finding and pseudo ranging to determine object location. However, the two approaches are applied separately, with the direction finding or angle of arrival approach being applied only to eliminate multipath errors from the location estimate.